Electrodeless high intensity discharge (EHID) lamps, in general, include an electrodeless discharge vessel containing a volatilizable fill material and a starting gas. The discharge vessel is mounted in a reflectorized fixture which is designed for coupling high frequency power to the discharge vessel. The high frequency produces a light-emitting plasma discharge within the discharge vessel. The applied electric field is generally colinear with the axis of the lamp capsule and produces a substantially linear discharge within the discharge vessel. The fixture for coupling high frequency energy to the discharge vessel typically includes a planar transmission line, such as a microstrip transmission line, with electric field applicators, such as helices, cups or loops, positioned at opposite ends of the discharge vessel. The microstrip transmission line couples high frequency power to the electric field applicators with a 180° phase shift. The discharge vessel is typically positioned in a gap in the substrate of the microstrip transmission line and is spaced above the plane of the substrate by a few millimeters, so the axis of the discharge vessel is colinear with the axes of the field applicators.
The electric field applicators used to deliver radio frequency (RF), or more particularly ultra-high frequency (UHF), power to the discharge vessel are separate units which for certain applications must be incorporated within the reflector used for harvesting the light from the EHID lamp. External tuning elements or elements embedding into the applicator must be used to deliver power to the lamp during all phases of glow-to-arc transition and plasma impedance swings. Openings need to be created in the reflector to accomodate the applicators thereby reducing the amount of reflective surface and the efficiency of the reflector to gather light, and in some cases weakening the physical integrity of the reflector. Applicators within the reflector volume also cause shadowing effects which are particularly acute in low-wattage EHID lamps where the size of the applicators is increased in proportion to the size of the discharge vessels.